[Elecraft] [K2] Tuning a magloop with KAT100

[Dave Lankshear]

Hello, Brian.

 

I've read your post and the replies and figure I maybe read what you said
incorrectly, but you did say:

 

Then use the TUNE button on the K2 to tune the loop to resonance, then
operate.

 

You can't use the K2's antenna tuner to tune a loop to resonance.  That's
not how they work.

 

The loop is a complete tuned circuit in itself.  There's the loop
representing inductance and a capacitor in parallel with it.  These are
resonant at a frequency and the only practical way to move that resonant
frequency is to adjust the value of the capacitor that's in parallel with
the loop.

 

You can't adjust that resonant frequency using an auto ATU at the end of a
length of coax.  It's rather like using a telephone conversation to make a
physical change at the other end.  You can't use the phone to make the beds
back home, when you're away!  The auto ATU will see the coax cable and the
loop as a lump of L and C and will endeavour to match it to 50 ohms to get a
1:1 SWR.  The coax forms part of the antenna and is not behaving like a
transmission line and the loop is not behaving like a resonant circuit - you
might just as well hook the coax to your automobile's fender and use the
auto ATU to tune that!

 

Please forgive my descriptions if I have misunderstood your question, but
re-reading your quoted statement makes me feel like I'm the only one who has
understood.

 

Yes, by all means match the auto ATU into 50 ohms using a dummy load.  That
way, the PA is looking into a load that matches the impedance of the
transmission line, although as Don suggested, you don't really need the auto
ATU (and its losses) as the PA should be reasonably well matched by
bypassing the auto ATU entirely.  Now, at the loop end, there's a gamma
match arrangement that ensures the loop, when at resonance, is a decent
match to the 50 ohm transmission line you're using.  So the rig matches to
the transmission line which matches to the loop that's been tuned to
resonance with its inbuilt tuning capacitor.

 

A loop is only a single turn coil, the resonant frequency of which is varied
by adjustment of its parallel capacitance.  Because it's small and is a low
loss inductor operating with a low loss airspaced (or vacuum) capacitor, the
Q factor, or "goodness" of the single turn coil at resonance is very high.
This means that a small excursion away from resonance, the loop's Q falls
very rapidly and renders it pretty useless, thus it is necessary (more so
when transmitting through the loop) to retune it for frequency shifts of
more than a few kHz.  That means that the SWR rises rapidly away from
resonance and the coaxial cable is more involved in becoming part of the
antenna and less of a transmission line.

 

The outer surface of a loop (well, outer 6%) needs to be of very low
resistance in order to maximise Q at resonance.  RF skin effect uses only
the outer surface of the conductor, thus the larger the surface area of the
conductor, the more its internal resistance is in parallel and thus reduced,
so the better performer the loop becomes.

 

Even a soldered joint on copper piping offers resistance that compromises
the loop's performance.  Recently a friend gave an old army magnetic loop to
a group of collectors/militia enthusiasts.  It was in poor condition, but in
its prime, its surface area must have been a foot across.  This makes a
mockery of the little bits of aluminium (aluminium) joined together with
bolts and wing nuts.  Yes, says the vendor, it is broad banded and only
needs retuning every 100kHz or so.  What he doesn't say is that its
resistance makesthe Q so appalling that its performance is lousy (where
lousy is the polite word), but those devices give properly engineered mag
loops a bad name by tarring all with the same brush.  Also, there are
proportionally more crappy mag loops out there simply because they are
cheaper than the "real" thing.  

 

The MFJ 1782/86/88 aren't too bad and are just at the crossing point between
good and bad, with a bias towards the good, if not too many spiders and
other insect life are resident under the black covers. These loops have an
airspaced variable capacitor within the black covers and that capacitor is
tuned by a small electric motor that's attached to it.  DC power for the
motor is fed down the coax cable itself, as well as the radio signals.  They
are not difficult to separate, eliminating the need for a control cable.
The DC voltage on the coax is reversed in order to make the motor turn in
the opposite direction.  The more sophisticated MFJ controller has an
inbuilt cross-needle SWR meter and the name of the game is to get the SWR as
low as possible on the operating frequency.  Their semi-automatic controller
drives the motor and detects the lowest SWR point and stops.  Of course, to
detect the lowest point, it has to begin to increase again, so it always
stops at a point that's not quite at resonance and this must be fine-tuned
by the operator, hunting to and fro with up/down press-buttons.  It takes
more effort to describe than to do in real life Hi!

 

One last caution.  Circulating currents are very high in a transmitting mag
loop and very high voltages (thousands of 'em) are also present when at
resonance, so ensure that the loop can't be touched by anything that matters
to you when it's transmitting, even at a few watts.  Give it a LOT of
respect, indeed, powerline respect.

 

I hope my efforts haven't missed the mark by a mile, Brian and that instead,
the wear and tear on my keyboard has been of some small use to you.

 

73 and early Season's Greetings.  Dave G3TJP